The use of optical fibers in communication systems is quite widespread and allows an increase in the speed of transmission and reduction in the size of the means used to carry the signals. However, in the current state of the art, a large portion of the processing performed on the information transmitted is carried out with electrical signals. There is, therefore a need to provide interfacing devices which allow coupling of the information sources and the receivers with the optical fiber segments.
In particular, the sources are provided with transducers that are capable of converting the electrical signals into optical relation to be launched into the fiber, while the receivers are provided with transducers that are capable of converting the optical radiation received from the fiber into electrical signals. In the former case, opto-electronic sources are used. These can include laser diodes or LEDs. The receiver transducers can be typically photo-detectors. The use of laser diodes is particularly widespread since the radiation emitted by these devices has advantageous characteristics for transmission through optical fiber. In particular, the optical radiation emitted is coherent and typically monochromatic, with the additional advantage that the small wavelength of such radiation allows the use of optical fiber cables with a smaller cross section by comparison with the cross sections of the cables carrying the radiation emitted by other sources. Once the optical source to be used has been chosen, the problem is how to drive it in such a way that, taking into account the source characteristics, appropriate optical signals sent through the fiber will correspond to the electrical signals emitted by the generator.
In laser diodes, the emission of optical radiation occurs only when the current flowing through the directly polarized device exceeds a minimum value, called the threshold current and hereinafter indicated as I.sub.s. The threshold current in general depends on various factors. Firstly it depends upon the level of technology and accuracy with which the laser diodes are manufactured, and among the devices manufactured with the same process, upon the inevitable spread of the actual characteristics with respect to the nominal value. In any case, it is possible to set the variation of the threshold current of the most commonly used laser diodes roughly between 5 mA and 30 mA.
Another characteristic of laser diodes is the fact that the power of the optical radiation emitted is directly proportional to the intensity of current flowing in the device in excess of the threshold current. This excess current is called the modulation current and hereinafter it is indicated by I.sub.m. Typical values for I.sub.m vary roughly between 0 mA and 30 mA. When information is to be sent in digital form, it is common practice to make a laser current, only slightly higher than I.sub.s, correspond to a logic level. In this way, the device is always on and, therefore, when switching from one logic level to the other, there is no delay due to the need to turn it on. A current given by the sum of Is and a certain value of I.sub.m is caused to correspond to the other logic level. This value of I.sub.m is proportional to the desired difference between the optical power associated with logic "1" and that associated with logic "0".
The difference is essentially chosen as a trade-off between the need to increase the margin of noise immunity (which corresponds to high levels of I.sub.m) and the need for fast switching (which is accomplished by reducing I.sub.m). Incidentally, by keeping the laser diode always above the threshold, the absence of an input signal corresponds to one of the two logic levels and no indefinite considerations can occur, which could cause a strong dissipation.
The laser diode is therefore a current-controlled device. Typically, however, generators supply the signals as voltages at logic levels. Thus, circuits are necessary which allow application of current I.sub.s to the laser diode and to convert the voltages received by the information generators into suitable modulation currents I.sub.m for superimposition on the current I.sub.s. Moreover, the driving circuits must allow the conversion of signals having the highest possible frequency, typically of the order of hundreds of MHz, minimizing dissipation of power.
As an alternative to the two approaches mentioned above, another technology can be used. It employs silicon and its basic component is the CMOS. Circuits in CMOS technology can have a high integration density, low power dissipation and low cost, but on the other hand their operating frequency is not very high and the power they dissipate is proportional to the square of the frequency. As an example of application of CMOS technology to driving circuits for optical sources, one can mention the driving circuit described by M. Steyaert et al in the document entitled "150 Mbit/s CMOS LED-driver and PIN-receiver IC for Optical Communication", presented at the IEEE 1992 Custom Integrated Circuits Conference. This publication presents a circuit, integrated in a single chip, for driving an LED at the frequency of 150 Mbit/s. The circuit has at its input side a cascade of CMOS inverters whose function is to couple the CMOS or TTL circuits upstream with the LED driving stage. This driving stage consists essentially of a current mirror circuit, which makes a bias current flow through the LED, the value of the current being imposed once and for all by dimensioning an external resistor, and of a transistor that controls the modulation current, arranged in parallel to the current mirror. This circuit has some drawbacks. For example, it works poorly at high frequencies (&gt;200 MHz), since under these conditions there is an accentuation in the phenomenon of the production of disturbances that originate on the switching fronts and propagate from the gate input of the transistor controlling the modulation current, toward the branch of the current mirror circuit that sets the threshold current. This brings about a reduction in the signal-to-noise ratio of the output of the laser diode, since the current peaks caused by the aforesaid disturbances reduce the dynamic range of the optical signal. Moreover, it is impossible to adjust the modulation current, while the nominal bias current remains rigidly fixed by means of the external resistor.